Multi-stage pyrolysis systems for treating chlorine contaminated wastes

ABSTRACT

A method of decontaminating solids contaminated with chlorinated hydrocarbons includes a first step of heating the contaminated solids at a temperature high enough to volatize chlorine contaminates but below a temperature range favorable to the formation of the dioxins and furans to dechlorinate the contaminated solids. Volatilized chlorine contaminates are removed from the dechlorinated contaminated solids. The dechlorinated contaminated solids are then purged with an inert gas to remove oxygen from the dechlorinated contaminated solids. Thereafter the dechlorinated contaminated solids are heated in the absence of oxygen to a temperature sufficient to crack hydrocarbons contaminating the solids to lower molecular weight hydrocarbons.

TECHNICAL FIELD

The present invention is directed toward an apparatus and method for thedestruction of hazardous chlorinated hydrocarbons, and more particularlyto a multi-stage pyrolysis system for treating solid wastes contaminatedwith chlorinated hydrocarbons.

BACKGROUND ART

The chemical weapons program of the U.S. military has lead to theproduction of thousands of tons of hazardous chemical warfare munitions.At the present time many of these weapons are being decommissioned bydisassembly and destruction. Incineration has been approved fordestruction of the chemical warfare agents, but concerns aboutgeneration of harmful byproducts has lead to reexamination of the wisdomof incineration.

In addition to destruction of chemical warfare agents, associated wastessuch as wood pallets, demilitarization protection ensemble (DPE) suits,contaminated activated carbon and miscellaneous wastes (cardboard,plastics, metals), collectively, “dunnage,” must be decontaminated.Under U.S. Army regulations, solid waste potentially contaminated withchemical warfare agents must undergo “5x” treatment. Treatment of wastematerials to 5x has been determined to result in solids free of chemicalwarfare agents and which are safe for handling by the general public. 5xtreatment requires solids to be heated to at least 1000° F. for at least15 minutes.

Unfortunately, many of the chemical warfare agents contain chlorineatoms. In addition, the dunnage may be contaminated with chlorine atoms.For example, the DPE suits comprise chlorinated organic polymers. Otherdunnage wastes may be contaminated with chlorinated organic compounds,for example pentachlorophenol which is used for insect resistance andpolychlorinated bi-phenol (PCB) compounds, both of which are toxic.Heating of the dunnage and chemical warfare agents containingchlorinated hydrocarbons in the presence of oxygen at temperaturesbetween 400-800° F. has been found to result in the production ofharmful dioxins and furans.

Several attempts have been made in the prior art to eliminate theproduction of harmful furans and dioxins during cracking or destructionof hydrocarbons in the presence of chlorine by removing chlorine atomsbefore high temperature incineration. For example, Zevenhoven and Saeed,Two-Stage Combustion of High-PVC Solid Waste with HCl Recovery,Proceedings of R′2000 World Congress on Recovery, Recycling,Reintegration, Toronto, Canada, Jun. 5-9, 2000, pp. 1212-1217(HTTP:www.rrr2000.com)—discloses a waste to energy process based onhigh-PVC solid wastes that includes a two-stage combustion of high-PVCsolid waste with hydrochloric acid recovery. In a first step, the PVCfraction is heated between 200-400° C. (396-750° F.) in the presence ofan inert gas (nitrogen) in an attempt to remove chlorine from residualsolids. The chlorine is removed in the form of a high temperature HCLgas. In a second stage the solids are incinerated in an oxidizingenvironment. Zevenhoven and Saeed teach that the removal of the chlorinebefore incineration limits dioxin/furan formation and removal of the hotHCL gas from the incineration minimizes corrosion. While Zevenhoven andSaeed teach the use of pyrolysis to remove chlorine molecules from theresidual solids, they teach this removal in the range of 200-400° C.,which is an optimum temperature range for the formation of harmfulfurans and dioxins. Thus, heating in this temperature range actuallyincrease the chances of dioxin formation. Subsequent incineration in thepresence of oxygen further enhances the opportunity for the formation ofdioxins and furans from any chlorine molecules that were notsuccessfully removed in the first stage.

In a like manner Kneko et al., U.S. Pat. No. 6,178,899, teaches thedesirability of limiting a supply of oxygen during pyrolysis of chlorinecontaining solids so as to minimize the risk of formation of dioxins andfurans. After a pyrolysis step the waste is subject to cracking in thepresence of oxygen in a temperature range of 1000-1200° C. Kneko issilent with respect to the temperature at which the pyrolysis isconducted. As with Zevenhoven and Saeed, if this pyrolysis is done attemperatures over 400° F., a risk of dioxin formation remainsnotwithstanding the absence of oxygen.

With the prior art recognized that heating chlorinating organics in theabsence of oxygen can remove chlorine compounds while minimizingproduction of undesirable dioxins and furans, the teachings of the priorart still run the risk of dioxin and furan formation from oxygenreleased from the heated hydrocarbons combining with chlorine moleculesat a temperature suitable for dioxin and furan formation to yieldundesirable dioxins and furans. Another problem faced by prior arttreatments for decontaminating chemical warfare munitions is validatingtreatment of the solids under the 5x conditions, i.e., heating to atleast 1000° F. for at least 15 minutes. Their problem results becausesolid wastes entering the unit can potentially pass through the unit ata rate faster than that predicted, causing an unacceptably shortexposure times.

SUMMARY OF THE INVENTION

A first aspect of the present invention is a method of decontaminatingsolids contaminated with chlorinated hydrocarbons. The method includesheating the contaminated solids at a temperature high enough to volatizechlorine contaminates but below a temperature range favorable to theformation of dioxins and furans to dechlorinate the contaminated solids.Volatized (gas phase) chlorine contaminates are removed from thedechlorinated contaminated solids. The dechlorinated contaminated solidsmay be purged with an inert gas to remove oxygen from the dechlorinatedcontaminated solids. Thereafter the dechlorinated contaminated solidsare heated in the absence of oxygen to a temperature sufficient to crackhydrocarbons contaminating the solids into lower molecular weighthydrocarbons. The heating of the dechlorinated contaminated solids belowthe temperature range favorable to the formation of dioxins and furansis preferably performed in the absence of oxygen. The solids may firstbe purged with an inert gas prior to volatilization of the chlorinecontaminates. In one embodiment the solids are heated to less than 400°F. Preferably the solids are heated in a range of between 300-400° F.The heating of the dechlorinated contaminated solids in the absence ofoxygen is preferably conducted at a temperature of at least 1000° F. forat least 15 minutes.

As second aspect of the present invention is a method of decontaminatingsolids contaminated with chlorinated hydrocarbons including purging thecontaminated solids with an inert gas to substantially remove oxygenform the contaminated solids. The purged contaminated solids are heatedat a first temperature high enough to volatize chlorine contaminants butbelow a temperature range favorable to the formation of dioxins andfurans to dechlorinate the contaminated solids. The volatized chlorinecontaminants are purged with an inert gas. The dechlorinatedcontaminated solids are heated to a second temperature sufficient tocrack hydrocarbons contaminating the solids into lower molecular weighthydrocarbons. The dechlorinated contaminated solids are purged with aninert gas to remove the gaseous cracked lower molecular weighthydrocarbons. The first temperature is preferably less than 400° F. Thesecond temperature is preferably greater than or equal to 1000° F. andthe dechlorinated contaminated solids are heated for at least 15minutes. The purged gas containing volatized chlorine contaminates ispreferably subjected to means for removal of the chlorinatedcontaminates to produce a dechlorinated purge gas. The dechlorinatedpurge gas may then be combined with the dechlorinated contaminatedsolids and heated to the second temperature to crack any residualhydrocarbons.

The method of decontaminating solids contaminated with chlorinatedhydrocarbons of the present invention decomposes the chlorinatedcontaminants at a temperature below that necessary for the formation ofdioxins and furans to ensure removal of the chlorine contaminants beforedioxins or furans can be formed. The additional step of performing thisfirst heating in the absence of oxygen (i.e., pyrolysis) furtherminimizes the possibility of the formation of dioxins and furans.Subsequent heating of the dechlorinated solids in an oxygen freeenvironment at or above the temperature necessary for the formation ofdioxins and furans minimizes the dioxin and furan formation because ofthe prior removal of chlorine and the absence of oxygen. Thus, the twostage pyrolysis process as described herein minimizes the potentialformation of dioxins and furans while still providing the necessarylevel of decontamination required to ensure an absence of hazardouschemical warfare agents.

BRIEF DESCRIPTION OF THE DRAWINGS

The FIGURE is a schematic representation of an apparatus for performingmulti-stage pyrolysis in accordance with the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

A multi-stage pyrolysis apparatus 10 is illustrated schematically in theFIGURE. Located upstream from the multi-stage pyrolysis apparatus 10 isan optional dunnage processor 12 for processing and delivering dunnageto the multi-stage pyrolysis apparatus 10. The dunnage processor 12includes an airlock chamber 14 containing a conveyor 16 for receivingwood products contaminated with chlorinated hydrocarbons, such as thepesticide pentachlorophenol. The airlock chamber 14 isolates the dunnageand possible harmful contaminates from the area of a building or groundswhere it is located. It further limits transport of gas phasecontaminants from one area to another. These contaminated wood productsare combined with other contaminated or potentially contaminated woodfrom a wood supply 18 and conveyed by conveyor 20 to a shredder 22. Theshredder 22 shreds the input wood material into small pieces suitablefor further processing. The shredder 22 includes a magnetic separator 24for separating nails, screws and other metal parts associated with thewood products and these metal parts are collected in the container 26.Shredded wood exits the shredder 22 and is deposited onto conveyor 28.Other contaminants may be combined with the shredded wood products forprocessing with the shredded wood products. For example, a supply ofspent activated carbon filters 30 may be delivered by conveyor 32 tohopper 34 and mixed with the wood products at the conveyor 28. Thecombined wood products and spent carbon are delivered to hopper 36 fromwhere they can be delivered as needed to bins 38 which are suitable forconveying the hydrocarbon contaminated solids through the multi-stagepyrolysis apparatus 10. Other solids may be added to the bin 38 prior topyrolysis treatment. For example, gas treatment solids from a supply 40or demilitarization protective ensembles (DPE) suits from a supply 42.

The multi-stage pyrolysis apparatus 10 consists of a series of airlockchambers suitable for receiving the bins 38 filled with contaminatedsolids and isolating the contaminated solids from the surroundingenvironment. More particularly, a charging airlock chamber 50 has afirst portal 52 suitable for isolating the charging airlock chamber 50from the ambient environmental and a second portal 54 suitable forportable an airlock between an adjacent low temperature pyrolysischamber 56. Adjacent to the low temperature pyrolysis chamber 56 is ahigh temperature pyrolysis chamber 58. The high temperature pyrolysischamber 58 is separated from the low temperature pyrolysis chamber 56 bya third portal 60 that selectively defines an airlock between the highand low temperature pyrolysis chambers. A fourth airlock portal 62selectively isolates the high temperature pyrolysis chamber 58 from adischarge chamber 64. A fifth airlock portal 66 permits access to thedischarge chamber 64 from the outside environment. In a preferredembodiment, a cooling/quenching chamber 68 receives a bin 38 from thedischarge airlock chamber 64. Thereafter, the decontaminated solids aredischarged for disposal.

An inert purge gas supply 70, for example nitrogen, is in fluidcommunication with the charging airlock chamber 50, the low temperaturepyrolysis chamber 56, the high temperature pyrolysis chamber 58 and thedischarge chamber 64 through a purge gas distribution system 72 whichdelivers the purge gas to a purge gas inlet 74, 76, 78, 80, associatedwith the charging airlock chamber 50, the low temperature pyrolysischamber 56, the high temperature pyrolysis chamber 58 and the dischargechamber 64, respectively. A purge gas outlet 82 is operativelyassociated with the charging airlock chamber 50. Purge gas outlet 84 isoperatively associated with the discharge chamber 64 to vent the purgegas to suitable cleaning apparatus such as a carbon filter 86.

Off gases from the low temperature pyrolysis chamber 56 are ventedthrough outlet 88 to an apparatus for removal of hydrochloric acid andchlorine gas 90. A conduit 92 delivers dechlorinated gas containinghydrocarbons to the high temperature pyrolysis chamber 58 to subject thehydrocarbons to cracking as will be described further below. An outlet94 from the high temperature pyrolysis chamber 58 delivers crackedhydrocarbons to a pollution control apparatus 96 for particulateremoval, converting carbon monoxide to carbon dioxide and for convertingvolatile organic compounds to carbon dioxide and water.

While the embodiment disclosed herein contemplates a single lowtemperature and a single high temperature pyrolysis chamber, there maybe as many pyrolysis treatment chambers as necessary to achieve therequired material processing rates.

The two-stage pyrolysis treatment method is carried out using thetwo-stage pyrolysis treatment apparatus as follows. A bin 38 ofcontaminated solids is delivered to the charging airlock chamber 50.Once inserted, the first portal 52 and the second portal 54 are closedto isolate the interior of the charging airlock chamber 50. The inertpurge gas is delivered by purge gas inlet 74 and functions to purge orremove substantially all oxygen gas from the charging airlock chamber50. Purge gas is exhausted through purge gas outlet 82 where it issubject to carbon filters for removal of hydrocarbon gases that may bedisplaced with the oxygen.

Following purging of oxygen in the charging airlock chamber 50, thesecond portal 54 is opened and the bin 38 is conveyed by a conveyor 98to the low temperature pyrolysis chamber 56. Once inside, the secondportal 54 is sealed and the contaminated solids are heated by heater 100to a temperature high enough to volatilize chlorine contaminates, butbelow a temperature range favorable to the formation of dioxins andfurans to dechlorinate the contaminated solids. The temperature ispreferably at about 300° F. and in any event less than about 400° F. Thecontaminated solids remain in the low temperature pyrolysis chamber 56long enough to substantially remove all chlorine atoms associated withthe contaminated solids. Purge gas delivered through the purge gas inlet76 maintains the low temperature pyrolysis chamber 56 oxygen free andconveys the chlorine gas and attendant hydrochloric acid gas vaporsthrough outlet 88 for delivery to the hydrochloric acid and chlorine gasremoval apparatus 90. The hydrochloric acid and chlorine gas removalapparatus 90 may be a wet scrubber or solid adsorbent. Cleaned gas canbe discharged to conditional air pollution control equipment for furthertreatment of volatile organic compounds or, as illustrated in theFIGURE, directed into the high temperature pyrolysis chamber 58 viaconduit 92 for cracking.

Following removal of chlorine atoms in the low temperature pyrolysischamber 56, the third airlock portal 60 is opened and the bin 38 isconveyed by conveyor 102 to the high temperature pyrolysis chamber 58.Thereafter the third portal 60 is sealed and the high temperaturepyrolysis chamber 58 is heated by heater 104 to a temperature sufficientto crack to the remaining hydrocarbon into lower weight molecularhydrocarbons. Where the method and apparatus is used for destruction ofchemical warfare agents and contaminated handling equipment thisrequires heating of the solids to a temperature of at least 1000° F. forat least 15 minutes. Cracked hydrocarbon gases are removed with thepurge gas through outlet 94 and delivered to pollution control apparatus96 for removal of particulate matter, conversion of carbon monoxide tocarbon dioxide and conversion of volatile organic compounds to carbondioxide and water vapor. Following pyrolysis in the high temperaturepyrolysis chamber 58, the fourth airlock portal 62 is opened and the bin38 is passed to the discharge chamber 64 by conveyor 106. Once sealedwithin the discharge chamber 64 additional purge gas is supplied viapurge gas inlet 80 and removed by purge gas outlet 84 for delivery tothe carbon filters 86. After a suitable retention time the fifth airlockchamber 66 is opened and the solids are exposed to ambient air withinthe cooling/quenching chamber 68. In the cooling/quenching chamber 68solids are quenched with water and cooled as desired. Thereafter the nowdecontaminated solids are disposed of.

It is important to note that within each of the chambers 50, 56, 58 and64, the solids are isolated from the adjacent chambers by the airlocksuntil they are passed to a subsequent chamber.

The preferred embodiment of the present invention contemplates a batchmode including transporting the contaminated solids in bins whichensures the process will not be short circuited. The method may also beemployed with continuous treatments systems using rotating internalmechanisms to transport the solids between adjacent pyrolysis zones. Onerepresentative rotating internal mechanism of this type is illustratedin Kaneko, U.S. Pat. No. 6,178,899, which is incorporated by referenceherein. However, such rotating internal mechanisms causing mixing orabrasion of the solids which can generate particulates which maystimulate dioxin and furan formation. Thus, while such an apparatus maybe used to practice the present invention and would be within the scopeof the claimed method, the batch apparatus described herein is preferredin part because it minimizes the formation of particulates duringprocessing.

As briefly described above, within the low temperature pyrolysischamber, chlorinated hydrocarbons are decomposed forming predominatelyhydrochloric acid with lesser amounts of chlorine gas and volatileorganic compounds. The exact mechanism for the pyrolysis ofpentachlorophenol is unknown, but is believed to be illustrated asfollows:

Pyrolysis of polychlorinated ethylene which comprises the DPE suits isalso not known, but is believed to be illustrated as follows:

The actual mechanisms may vary somewhat from that shown above. Otherchlorinated hydrocarbons are decomposed to release chlorine gas,hydrochloric acid and volatile chlorinated organics in a similar mannerfor removal from the contaminated solids.

Within the high temperature pyrolysis chamber 58 high molecular weighthydrocarbons are cracked through exposure to high temperatures in theabsence of oxygen to form carbon monoxide, carbon dioxide and lowermolecular weight hydrocarbon gases.

The two state pyrolysis apparatus and method described above is intendedto remove elements which are necessary to the formation of dangerousdioxins and furans during heating and cracking of the contaminatedwaste. Key variables that have been identified in the prior art asaffecting dioxin and furan are particulate material in the gas stream,transition metals that are exposed from corrosion of metal ducting orare associated with combustion fly ash, HCl and chlorine gasconcentration, an oxidative environment, and, perhaps most important, afavorable temperature range of 400-850° F. The initial oxygen purging inthe charging airlock chamber 50 substantially removes oxygen gas fromthe contaminated solids. Within the low temperature pyrolysis chamber 56dioxin or furan production is prevented both by the absence of oxygennecessary for oxidation by virtue of the continuous purging with aninert gas such as nitrogen and by temperatures below that necessary forthe formation of dioxins and furans. This permits the removal ofchlorine compounds which otherwise may lead to dioxin and furanproduction in the high temperature pyrolysis chamber 58. Processing in abatch mode using the preferred apparatus described herein also minimizesthe formation of particulate matter. Within the high temperaturepyrolysis chamber 58 two primary factors again prevent or minimize theformation of dioxins and furans. First, as discussed above thecontaminated solids are dechlorinated in the low temperature pyrolysischamber 56. In addition to the absence of chlorine atoms in the gasphase in the chamber 58, the inert purge gas 78 removes oxygenatedorganic compounds that may result from the cracking of the largemolecular weight hydrocarbons. Thus, the substantial absence ofchlorinate gases and the substantial absence of oxygen gas necessary foroxidation effectively prevents dioxin and furan production,notwithstanding the potential of favorable temperatures in the range of400-850° F. during heating and cooling of the solids as they enter andleave the high temperature pyrolysis chamber. Production of dioxins andfurans are further discouraged by removal of highly corrosive HCl andchlorine gases before the high temperature pyrolysis. Removal of thesegases minimizes corrosion of the metal components which could createtransition metals favoring dioxin and furan production. Moreover, asmentioned above, the preferred batch process minimizes formation ofparticulate matter known to simulate dioxin and furan formation.

The present invention is particularly beneficial for the decommissioningof chemical warfare agents and the decontaminating of handling equipmentand meets the governments required 5x treatment requiring temperaturesof 1000° F. for at least 15 minutes. The two stage pyrolysis systemassures that in both the low temperature pyrolysis chamber 56 and thehigh temperature pyrolysis chamber 58 at least two of the necessarycondition for the formation of dioxins and furans are not present. Thus,the two stage pyrolysis apparatus and method provides the requireddecontamination of the treated solids without the dangerous ofproduction of dioxins and furans.

1. A method of decontaminating solids contaminated with chlorinatedhydrocarbons comprising: a) heating the contaminated solids at atemperature high enough to volatize chlorine contaminates but below atemperature range favorable to the formation of dioxins and furans todechlorinate the contaminated solids; b) removing the volatilizedchlorine contaminates from the dechlorinated contaminated solids; and c)cracking hydrocarbons contaminating the solids into lower molecularweight hydrocarbons by heating the dechlorinated contaminated solids inthe absence of oxygen to a temperature sufficient to crack hydrocarbons.2. The method of claim 1 wherein step a) is performed in the absence ofoxygen.
 3. The method of claim 1 wherein prior to step a) thecontaminated solids are purged with an inert gas and the heating of stepa) is conducted while purging the contaminated solids with an inert gas.4. The method of claim 1 wherein step b) is performed by purging thevolatilized chlorine contaminates from the contaminated solids with aninert gas and then subjecting the purge gas containing the volatilizedchlorine contaminates to means for removal of the chlorinatedcontaminates to produce a dechlorinated purge gas.
 5. The method ofclaim 4 further comprising flowing the dechlorinated purge gas to theheating step d) to subject any residual hydrocarbons in thedechlorinated purge gas to cracking.
 6. The method of claim 1 whereinthe heating temperature of step a) is less than 400° F.
 7. The method ofclaim 1 wherein the heating temperature of step a) is between 300-400°F.
 8. The method of claim 1 wherein the heating temperature of step d)is at least 1000° F. and the dechlorinated solids are exposed to thetemperature of at least 1000° F. for at least 15 minutes.
 9. The methodof claim 1 wherein the solids reside in a container during steps a)-d).10. The method of claim 1 further comprising before step c) purging thedechlorinated contaminated solids with and inert gas to remove oxygenfrom the dechlorinated contaminated solids.
 11. A method ofdecontaminating solids contaminated with chlorinated hydrocarbonscomprising: a) purging the contaminated solids with an inert gas tosubstantially remove oxygen gas from the contaminated solids; b) heatingthe contaminated solids at a first temperature high enough to volatilizechlorine contaminates but below a temperature range favorable to theformation of dioxins and furans to dechlorinate the contaminated solids;c) purging the contaminated solids with an inert gas to remove thevolatilized chlorine contaminates; d) cracking hydrocarbonscontaminating the solids into lower molecular weight hydrocarbons byheating the dechlorinated contaminated solids to a second temperaturesufficient to crack hydrocarbons and e) purging the dechlorinatedcontaminated solids with an inert gas to remove gaseous cracked lowermolecular weight hydrocarbons.
 12. The method of claim 11 wherein thefirst temperature of step b) is less than 400° F.
 13. The method ofclaim 12 wherein the second temperature of step d) is greater than orequal to 1000° F. and the dechlorinated solids are heated for at least15 minutes.
 14. The method of claim 11 further comprising subjecting thepurge gas containing the volatilized chlorine contaminates to means forremoval of the chlorinated contaminates to produce a dechlorinated purgegas.
 15. The method of claim 14 further comprising flowing thedechlorinated purge gas to the heating step d) to subject any residualhydrocarbons in the dechlorinated purge gas to cracking.